The person completing the action, i.e. the subject, is not indicated in the suffix (ending) of an impersonal verb.

-ir, -wyd and -id are the most commonly used suffixes for impersonal verb forms.  

The impersonal verb forms can be shown as follows:

 

-ir	Gwelir	Present Tense/Future	Gwelir clychau'r gog bob mis Mai.
-wyd	Gwelwyd	Past Tense	Gwelwyd eira yn ystod y gaeaf llynedd.
-id	Gwelid	Imperfect Tense	Gwelid gwiwerod coch mewn coedwigoedd ers talwm.

Cael is an irregular verb.  The impersonal form of the past tense is cafwyd, e.g.

Cafwyd glaw trwm yn y dref ddoe.

Cafwyd tair gwobr gyntaf yn yr eisteddfod.

 

The normal sentence structure in Welsh is Verb + Subject + Object, e.g.

Rhedodd y ferch y ras.

Ysgrifennodd y bardd gerdd.

Dylai'r plant gael gwersi.

 

However, sentences which include impersonal verbs do not follow this pattern. 

When using the impersonal tense, the object follows the verb and the subject/person completing the action is included after gan, e.g.

Rhedwyd ras gan y ferch.

Ysgrifennwyd y gerdd gan y bardd.

Note also that there is no soft mutation after an impersonal verb (unlike personal verbs), e.g.

Ysgrifennodd y bardd gerdd.

Ysgrifennwyd cerdd gan y bardd.